1. Field of the Invention
The present invention relates to a thermal head used in thermal character recording.
2. Description of the Prior Art
A thermal head consists of, for example, a glazed ceramic substrate and a plurality of heating resistors formed thereon. Conductors for supplying electric power to the heating resistors are also disposed on the glazed ceramic substrate. Recording information with this thermal head is performed in the following manner. A current is supplied through the conductors to the predetermined heating resistors which are necessary for obtaining a heat pattern corresponding to information to be recorded, thereby heating the heating resistors. Then, the heating resistors are brought into contact with a recording medium, thereby recording the information.
A heating resistor of thick film type is known as such a heating resistor, as reported in Japanese Patent Disclosure No. 54-44798. In order to obtain the heating resistor of this type, a resin and a solvent are added to a mixture of RuO.sub.2 and glass making a paste which is coated and baked on a substrate. However, this heating resistor has large variations of resistance due to the particle size of the raw material, scatter temperature and time. In addition, since the heating resistor of this type is manufactured by screen printing which cannot substantially perform micropatterning, the resolution becomes undesirably low.
On the other hand, a heating resistor of the thin film type formed of tantalum nitride, nichrome, Cr-Si series cermet and the like is known. However, when the heating resistor formed of such a material is heated to a high temperature, considerable oxidation occurs, thereby increasing its resistance. Generally, in order to prevent such a disadvantage, an oxidation resistance film formed of SiO.sub.2 is provided on the surface of the heating resistor. However, oxidation cannot be sufficiently prevented and the manufacturing procedures become overly complex if this oxidation resistance film is formed. In particular, since the SiO.sub.2 film has a low thermal conductivity, it takes considerable time to transmit heat energy from the heating resistor to thermal paper through this SiO.sub.2 film. Therefore, it is difficult to realize the type of high-speed printing that has been demanded recently. On the other hand, the difference between the thermal expansion coefficients of the substrate and the heating resistor causes cracking in the heating resistor in time, resulting in a malfunction of the thermal head. Furthermore, in order to prevent degradation due to oxidation, the heating resistor must be maintained at a low temperature, and so the high-speed printing demanded recently cannot be realized.
A heating element formed of SnO.sub.2 in which an impurity is doped is known. However, this material is known as an oxide semiconductor, and has the characteristic that the resistance value is decreased in accordance with an increase in temperature. If the heating resistor having such a characteristic is used, the initial value of the electric power must be decreased because of the low voltage-current capacity of the semiconductor to which electric power is supplied and controlled, resulting in a low printing speed.
The present invention has been made in consideration of the above situation, and has as its object to provide a thermal head which can realize high-speed printing and can be stable over time.